1. Field of the Invention
The present invention relates to a construction machine, and, in particular, relates to a rotary digging and compression enlarging driller with a function of boring and enlarging a hole. The driller is capable of boring a straight hole by boring and capable of forming an enlarged portion in the straight hole by rotary compression enlarging. The present invention further relates to a boring method using the driller.
2. Description of the Related Art
There are various drillers for boring pile shafts in foundations. Most conventional drillers can bore only straight holes. A rotary digging driller is proposed in recent years. An enlarging tool for enlarging a bottom of a pile shaft may be provided at an end of a drilling rod of the driller, but the enlarging tool can enlarge only a bottom of a bored straight hole. Furthermore, during enlarging the bottom of the bored straight hole, it is necessary to remove a boring tool from the drilling rod and to attach the enlarging tool to the drilling rod. In addition, the enlarging tool enlarges the bottom of the bored straight hole by cutting.
Currently, a widely applicable technique is proposed in which a multi-node pile can be constructed by compression enlarging of a pile shaft and poring concrete into the pile shaft with enlarged portions. Specifically, enlarged portions are formed at a plurality of predetermined positions in a bored straight pile shaft so that a pile can be formed to have a plurality of enlarged portions by filling the shaft with concrete. The technique can greatly improve the bearing capacity of a single pile, and decrease the number of required piles, the amount of required concrete, and the cost.
A prior process for constructing a pile by the above technique is carried out in the following manner. First of all, a straight hole is bored by a driller, and a plurality of enlarged portions are formed at a plurality of predetermined positions in the straight hole by a separate special compression enlarging apparatus. Then, the straight hole is cleaned and the diameter of the straight hole is detected. A reinforcing steel bar cage is placed into the straight hole. Finally, concrete is poured into the straight hole
The above conventional construction process can improve the bearing capacity of a single pile, but has the following disadvantages. The number of steps of construction process, the cost and time for constructing a single pile increase. Since a compression enlarging is carried out by compression enlarging, rotating through an angle, and compression enlarging again in sequence, soil around the enlarged portion is disturbed when the enlarged portion is formed in the straight hole, so that spoil tends to drop on a bottom of the shaft and enlarged portion is not regular. If an enlarged portion is formed in a straight hole by cutting soil mass surrounding the shaft with a separate rotary digging and compression enlarging driller provided with a bottom-enlarging tool at an end of a drilling rod thereof, one enlarged portion can only be formed at the bottom of the straight hole. As a result, during forming the enlarged portion by cutting the surrounding soil mass, the soil body surrounding the cut soil mass is perturbed, but is not packed. Therefore, a side frictional resistance and an end resistance applied to the pile in the straight hole can not act on the pile together, so that the pile can not bear a high load.